The present invention relates to an apparatus for driving an optical sensor constituted by a charge-coupled device (hereinafter referred to as CCD-sensor) and used in facsimile equipment and the like. More particularly, the invention concerns a CCD-sensor driving apparatus which is arranged such that the quantity of light emitted by a light source is detected to thereby determine the storage time of the CCD in dependence on the detected quantity of light, whereby the CCD-sensor is driven periodically at a time interval corresponding to the determined storage time so that output signal resulting from photoelectric conversion effected by the CCD-sensor may remain steady independent of variations in the quantity of light of the light source.
The CCD which has come into the world as a development of the MOS technology is generally categorized into a charge-transfer device which term also covers a bucket brigade device (BBD) and others. In contrast to the semiconductor devices such as bipolar and unipolar elements whose operation is based on the thermally balanced condition, the concept of operation of the CCD is unique in that mobile charge (minority carriers) is shifted between temporary storage sites referred to as the potential-energy wells in the thermally balanced state. By virtue of this unique operation, the CCD is made use of in a wide variety of applications such as imagers, facsimile equipment and others.
FIG. 1 of the accompanying drawings shows in a block diagram an arrangement of a reader system for a facsimile equipment which incorporates a fluorescent lamp as the light source together with a CCD-sensor driving apparatus of a hitherto known structure. Referring to the figure, a document 1 containing information is illuminated with a fluorescent lamp 2 and light information is directed to a CCD-sensor 4 by way of a lens system 3 to undergo photoelectric conversion. The CCD-sensor 4 responds to a start signal (START) produced by a start pulse generator 5 to output an electric information signal which is then applied to an inverting input terminal of an operational amplifier 7 through a resistor R.sub.1. Connected in parallel with the operational amplifier 7 is an automatic gain control circuit 6 which operates in response to the output of the CCD-sensor 4 to stabilize the video output signal (VIDEO) of the operational amplifier 7. FIG. 2 shows signal waveforms for illustrating operation of the apparatus shown in FIG. 1. Referring to FIG. 2, every time the pulse signal "START" is produced by the start pulse generator 5, a signal representative of information of the document 1 resulting from photoelectric conversion is obtained from the output of the CCD-sensor 4. It should be noted that the pulse signal "START" is produced at a predetermined fixed period or time interval t.sub.1. The waveform 2a shown in FIG. 2 represents the output signal of the CCD-sensor 4 produced when the document 1 being read under illumination with a standard quantity of light of the fluorescent lamp 2 is either white or black. The waveform 2b represents the output signal of the CCD-sensor 4 produced upon reading the white areas of the document under illumination with a decreased quantity of light, while the waveform 2c represents the output signal of the CCD-sensor 4 produced upon reading black areas of the document under illumination with the standard quantity of light. It will be seen that peak values V.sub.p2 and V.sub.p3 of the signals 2b and 2c are smaller than the peak value V.sub.p1 of the waveform 2a, i.e. V.sub.p2 &lt;V.sub.p1 and V.sub.p3 &lt;V.sub.p1. For these waveforms 2a, 2b and 2c, the output signal "VIDEO" of the operational amplifier 7 assumes a peak value V.sub.p0 under the control of the AGC circuit 6. As the consequence, there may arise such an undesirable situation in which white and black information is confused with each other to bring about erroneous discrimination because of the fact that both information (waveform 2b) read from the white portion of the document under illumination with a decreased quantity of light and information (waveform 2c) read from the black portion of the document under illumination with the standard quantity of light take the utterly same peak level V.sub.p0 in the signal "VIDEO". This is a serious disadvantage of the hitherto known apparatus shown in FIG. 1.